Fiber optics and light have been used to measure hazardous materials like hydrogen gas, where a safe and accurate detection is essential. Because fiber optic technology employs non-reactive light guided inside a fiber, permitting remote, real-time measurement implementation, it can be used in explosive/hazardous environments such as hydrogen leak detection.
Other current, commercially available technologies used to measure hydrogen concentrations are primarily electrical in nature. Some examples are: Hydrogen Field Effect Transistors (HFETs) such as Metal Oxide Semiconductor (MOS), Metal Insulator Semiconductor (MIS) Field Effect Transistors and semiconductor sensors20-22 such as thin film resistors. Electrical means of detection are inherently dangerous because of the fact that they use electrical current near a potentially explosive environment to detect hydrogen. Furthermore, they cease to function as intended at cryogenic temperatures.
Conventional Extrinsic Fabry-Perot fiber optic sensors based on the interference of the two or more light waves from separate multiple light sources that are reflected from the two reflective surfaces at the end of the Fabry-Perot cavity can be used to measure various measurands such as strain, force, temperature, pressure, etc. The measurand effect on the sensor changes the cavity length, producing a change in the phase of the reflected light and the interference patterns of the reflected light as an interferometer output signal. The output signal is a sinusoid generated by the two reflected beams moving in and out of phase with respect to one another. The number of sinusoidal cycles (fringes) in the output signal is proportional to the change in the length of the cavity for a given wavelength of light. Since the change in length of the cavity is related to the measurand, the number of fringes produced can be correlated to the parameter measured, such as temperature.
The conventional sensor described above requires that the temperature of the sensing point be held constant if another parameter is being measured. This requires heating or cooling of the sensing point, for example, the temperature can be maintained by means of a relatively high power laser. Such applications require separate monitoring of the temperature at the sensing point. Where two separate independent variable parameters are measured, for example temperature and hydrogen, a temperature sensor probe is used with the hydrogen sensor probe. When the two independent variables need to be measured by a single sensor, two wavelengths (i.e., an additional light source) must be employed. This is the case where temperature affects the gap length due to expansion and contraction of the materials. In addition to the added cost and complexity, the dual or multiple wavelength method is usable only over a limited range before the two variables cannot be separated (least common multiple of the wavelengths).
These disadvantages of conventional systems and methods for measurement using electricity or light are overcome by the invention described in this application, as would be understood by those skilled in the art by reading the following description of the invention.